The invention relates to an electron gun for a linear accelerator, which makes it possible to modulate the electronic current as soon as it is formed. It also relates to an accelerating structure equipped with such an electron gun.
Electron guns, e.g. of the triode type, are used for generating electrons injected into an accelerating structure of a linear accelerator. An electromagnetic wave of an appropriate frequency for the accelerating structure makes it possible to accelerate these electrons and produce a beam, whose characteristics vary as a function of its application.
These applications are varied and linear accelerators are used for scientific purposes or for forming irradiators intended for use in therapeutic treatment and even more frequently for forming industrially used irradiation machines. These industrial irradiation machines e.g. make it possible to sterilize various products and can generally produce electron beams having energy levels between 1 and 10 MeV.
The electrical power consumed by these machines is considerable and can e.g. reach 130 Kw, of which only 20 Kw is in the accelerated electron beam. In addition, the overall efficiency of such a machine has significant direct repercussions on the cost of treating the products. Bearing in mind the industrial nature of these operations, and the importance of the electric power consumed, an improvement in this efficiency has very favourable economic consequences.
It is known to group electrons into clusters by means of so-called bunchers located on the path of the electrons, the bunchers being positioned between the electron gun and the accelerating structure into which these clusters are injected. Generally, the electronic current supplied by the gun is established in a continuous manner, during a given, repetitive operating time. The electrons constituting this current are grouped by these bunchers into clusters having a given duration and period by the operating characteristics of said bunchers.
Such a grouping of electrons into clusters may make it possible to improve the efficiency of a linear accelerator, to the extent that it obviates the consumption of the energy carried by the electromagnetic wave for electrons located outside of the range of the in phase acceptance of the accelerator, but it is difficult and complicated to realize such bunchers.
In the present invention, an improvement to the overall efficiency of a machine using an accelerating structure is obtained by means of a modulation of the electronic current, effected as soon as the latter is generated by the electron gun. For example, this modulation makes it possible to obtain clusters of electrons injected into the accelerating structure, without requiring bunchers for this purpose.
Electron guns for linear accelerators are frequenty constituted by a thermionic cathode and an anode, between which a grid controls the electronic current, as a function of a potential difference established between the grid and the cathode. This potential difference can be continuous for determining an operating time, in the manner explained hereinbefore, or within this operating time can have variations at high frequencies or brief transience. In the latter case, it constitutes a signal, which is generally processed by a complex injection modulator and carried by a transmission line having appropriate characteristics. However, the establishment of this signal between the grid and the cathode relative to the signal processed by the injection modulator causes difficulties, more particularly as a result of stray capacitances.